Getting isolation from the mains supply is an on-going challenge for me. Here's my latest stab at getting as close to a battery as I can currently manage - a ferrite line-output trafo soon to be pressed into service as a low-capacitance isolation transformer.

The back story is that I'd heard that split bobbin trafos were the way to go for the best isolation from the mains. Having just acquired some for chipamp-drive unit interfacing I was curious to check out their credentials. I measured around 20pF interwinding capacitance which is about the lowest of any mains trafo I've checked. Of that it seems that the majority of the capacitance is directly between the windings, a smaller amount is due to coupling via the steel core (which of course is conductive). So then I figured that even if the windings could be moved infinitely far apart the steel core would be the limiting factor in reducing the capacitance.

The 1.8mm iron wire arrived and it did indeed have a 1.2mm internal diameter which is perfect for winding small chokes. I built a second box with steel chokes in but the third box I tried with the iron wire coils. Having insulated wire means the coils are smaller and I was able to fit the second tier filter into the same box.

I wound a couple of NiZn toroids with iron wire, but not as common mode chokes. When I did the sums I found that for the currents I'm running at (under 3A total draw) the toroidal core isn't close to saturation. Hence I decided on separate inductors to gain some differential mode filtering on top of the common mode.

The next stage of development has been trying to wind multilayer iron wire air-cored chokes. For this I've had trouble finding formers which allow more than 4 layers. The reason for wanting more layers is - this makes the chokes much more lossy. Proximity effect helps concentrate the higher frequency currents only in areas...

OK, slight change of plan. Due to a ****-up on the ordering front with the iron wire, I ended up with some wire which is a bit too thin (0.5mm inner diameter) as they specified the outer diameter on the ordering page, rather than the conductor diameter. On reflection of course, that makes perfect sense since this isn't wire for electrical purposes at all, what use is the 'conductor' diameter? Wire with an external diameter of 1.8mm's on its way, which I hope will have ~1.2mm diameter conductor.

So, in the meantime I've been experimenting with wire I have been able to get hold of, which is shiny steel. It turns out that only 30 paces from my home is a hardware store which sells it - so I've tried winding chokes with it, just to see what happens. I've also acquired an LCR meter to see what kinds of values I'm getting at different measuring frequencies.

The first pair of chokes I wound with some really thick stuff - 2.8mm. Having aimed for 20 turns...

I've been actively researching via Google on this project since the last posting and found out a few interesting things which I'll share here.

Firstly I checked out piano wire as the material for making the L1 & L2 input side chokes in my original schematic. I couldn't find any consistent figure for what the permeability of piano wire is - it seems steel comes in so many variants that its hard to tell. So I continued to look for alternative materials which have reasonably high permeability. Remember, its the high permeability which gives rise to skinny skin depths and hence higher losses as the frequency climbs. Normally this is really undesirable, but for our purposes its exactly what we're looking for.

Other elements which have high permeability include nickel, cobalt and iron. Nickel wire is available, but its expensive and not widely sourced. Cobalt is even more expensive with even fewer sources. Iron is the cheapest option and it turns out that iron...

Since Ostripper asked about mains filtration on the RF & Audio thread, here are some notes and ideas. I haven't built any of this yet - if you decide to, I'll be interested in any results - photos and listening reports. Be careful with this stuff, its at mains potential.

Pre-built mains filters are of some use in audio, but they're not really optimised for high-end sound. For audio source components, the draw is normally under 100W and so the current (assuming here 230V supply) is under half an amp. Few if any off the shelf mains filters are designed for such low currents.

Second, the commercial filters assume that we wish to stop conducted interference getting out as much as prevent muck getting in. The internal noise would normally only come from the rectifiers in a linear power supply (SMPSUs are a totally different matter). Once we've snubbed them (you have done that already right, or are using soft-recovery or schottky diodes?), with a linear...